1. Field of the Invention
The present invention relates to a control device for power amplifiers which are used in cellular radio telephones, portable telephones and the like and which need to change output power thereof.
2. Description of the Prior Art
FIG. 11 is a block diagram of a conventional automatic output power control circuit. In the figure, reference numeral 1 represents an amplifier to be controlled, 2 an output detector for detecting the output of this amplifier 1, 10 a variable voltage generator circuit for changing output power through resistance division, 4 a D.C. amplifier for amplifying the difference between the detection output of the output detector 2 and the variable output voltage of the variable voltage generator circuit 10, and 5 a source current control circuit for controlling the source current of the amplifier 1 with the output of the D.C. amplifier 4 to change the output of the amplifier 1.
In a car telephone, for example, output power is limited to 8 levels up to 4.0 W in increments of 4 dB, and a mobile station needs to change its output power to a level prescribed by a fixed station.
Next, the operation of the control device will be described. The output of the amplifier 1 is detected by the output detector 2 and the detected output is supplied to the D.C. amplifier 4. To this D.C. amplifier 4, an output voltage from the variable voltage generator circuit 10 is also applied. The difference between these voltages is measured by the D.C. amplifier 4 and this differential voltage is transmitted to the source current control circuit 5. This source current control circuit 5 changes the source current of the amplifier 1 according to this differential voltage in order to change the amplifier output power. In this way, the output power of the amplifier 1 can be controlled by changing the output voltage of the variable voltage generator circuit 10.
Since the conventional automatic output power control circuit is structured as described above, it induces such problems as amplifier overshooting, amplifier emission of an undesired spectrum, and the like when amplifier output power is turned ON and OFF, or amplifier output power is changed, because the output of the variable voltage generator circuit may be constant or vary stepwise.